The present invention relates to microstrip antenna structures and more specifically to a microstrip antenna having wide bandwidth characteristics (greater than about 20% with a VSWR of 2:1 or less) and which employs slot, i.e., aperture coupling.
The use of microstrip techniques to construct microwave antennas has recently emerged as a consequence of the need for increased miniaturization, decreased cost and improved reliability. One primary application of high interest is in the construction of large phased array systems.
However, microstrip antennas have heretofore suffered from relatively narrow operational bandwidth, which limits tunability of the devices. It is desirable to have an antenna having at least as great a bandwidth as the feed system. And it is in general desirable to have devices with as wide a bandwidth as possible for various wideband applications.
The following references were uncovered in relation to the subject invention:
Pozar, D.M., "Microstrip Antenna Aperture-Coupled to a Microstripline," Electronics Letters, Vol. 21, pp. 49-50, January 1985, describes an aperture coupling technique for feeding a microstrip antenna. While the basic aperture feed technique appears similar to that of the subject invention, there is no suggestion of how to achieve a wide continuous bandwidth.
Yee, U.S. Pat. No. 4,329,689 describes a microstrip antenna structure having stacked microstrip elements. However, a second type of coupling is employed. The coupling is a direct, mechanical connection. A central conductor extends from the ground plane directly to the uppermost conducting plane which serves as a radiator. Because there is a central conductor extending through the multiple layers, the center conductor presents an inductance which contributes to detuning effects, an undesirable characteristic. Physical connection such as soldering is required to secure the feed electrically to the conducting plane. Couplings which rely on physical connection are subject to undesired mechanical failure. No provision is shown or suggested for continuous wideband operation.
Fassett et al., U.S. Pat. No. 4,554,549 describes a microstrip antenna with a third type of feed. therein a feedline and a radiating element, a ring, are on the same side of a ground plane. As a consequence, there is a possibility that undesired or stray radiation patterns may be generated from the feedline.
Black, U.S. Pat. No. 4,170,013 describes an antenna with a stripline feed, rather than a microstrip feed. The stripline is sandwiched between two ground planes and directly connected to a radiating patch. The radiating patch in turn radiates through an aperture. The aperture must be larger than the radiating patch. The device is basically a stripline structure.
Bhartia, U.S. Pat. No. 4,529,987 describes a microstrip antenna having a bandwidth broadening feature in the form of a pair of varactor diodes. Physical connection of the diodes is required to electrically couple between the radiator and the ground plane.
Lopez, U.S. Pat. No. 4,364,050 describes a microstrip antenna wherein the radiating elements are cross-slots in a conducting sheet sandwiched between a vertical feed network and a horizontal feed network. Interference may result in the radiation pattern because of blockage and feed network radiation.
I-Ping Yu, "Multiband Microstrip Antenna," NASA Tech Briefs, Spring 1980, MCS-18334, Johnson Space Center, describes a multiband, narrow bandwidth microstrip antenna having a direct physical connection between radiating elements and a pin feed attached to a coaxial connector. No provision is made for providing continuous wide-bandwidth operation.
Sabban, A., "A New Broadband Stacked Two-layer Microstrip Antenna," Digest, 1983 IEEE AP-S International Symposium, May 23-26, pp. 63-66, 1983 (CH1860-6/83) describes still another microstrip antenna which employs a direct feed. The design described is said to have a continuous bandwidth of 9-15 percent. However, the microstrip feedline resides on the same surface as the "feeder element" and is in direct connection with patches, a different configuration as compared to the present invention.
Chen et al., "Broadband Two-layer Microstrip Antenna," Digest, 1981 IEEE AP-S International Symposium, pp. 251-254, 1984 (CH2043-8/84) describes still another microstrip antenna with a direct feed. A probe, which is typically the center conductor of a coaxial cable is connected as by soldering to a first patch near the ground plane. As such, the physical connection is subject to failure, and the probe presents an effective inductance which contributes to detuning effects.
James et al., Microstrip Antenna Theory and Design, IEE, 1981: Peter Peregrinus Ltd., Chapter 10 (on trends and future developments) illustrates various schemes for a patch antenna. Of particular note is FIG. 10.18 on page 274, which shows a slot aperture. Significantly, there is no structure above the ground plane wherein the slot resides. The feed method is such that the aperture itself serves as a radiator, and is thus a slot antenna rather than an aperture antenna.
United Kingdom Patent Application No. GB 2,166,907 A describes still another microstrip antenna in which there is a direct coupling to a radiating element. Therein the device is tuned without significantly affecting bandwidth by painting coatings of a dielectric across the radiating surface. This is a fabrication technique for producing a pretuned conventional narrow bandwidth microstrip antenna.
What is needed is a microstrip antenna having a physically-robust coupling and which is capable of wideband operation.